Electro-mechanical spatial light modulators have been designed for a variety of applications, including image processing, display, optical computing and printing. Optical beam processing for printing with deformable mirrors has been described by L. J. Hornbeck; see U.S. Pat. No. 4,596,992, issued Jun. 24, 1984, entitled "Linear Spatial Light Modulator and Printer". A device for optical beam modulation using cantilever mechanical beams has also been disclosed; see U.S. Pat. No. 4,492,435, issued Jan. 8, 1985 to M. E. Banton, entitled "Multiple Array Full Width Electro-Mechanical Modulator," and U.S. Pat. No. 5,661,593, issued Aug. 26, 1997 to C. D. Engle, entitled "Linear Electrostatic Modulator". Other applications of electromechanical gratings include wavelength division multiplexing and spectrometers; see U.S. Pat. No. 5,757,536, issued May 26, 1998 to A. J. Ricco et al., entitled "Electrically Programmable Diffraction Grating".
Electro-mechanical gratings are well known in the patent literature; see U.S. Pat. No. 4,011,009, issued Mar. 8, 1977 to W. L. Lama et al., entitled "Reflection Diffraction Grating Having a Controllable Blaze Angle," and U.S. Pat. No. 5,115,344, issued May 19, 1992 to J. E. Jaskie, entitled "Tunable Diffraction Grating". More recently, Bloom et al. described an apparatus and method of fabrication for a device for optical beam modulation, known to one skilled in the art as a grating-light valve (GLV); see U.S. Pat. No. 5,311,360, issued May 10, 1994, entitled "Method and Apparatus for Modulating a Light Beam". This device was later described by Bloom et al. with changes in the structure that included: 1) patterned raised areas beneath the ribbons to minimize contact area to obviate stiction between the ribbon and substrate; 2) an alternative device design in which the spacing between ribbons was decreased and alternate ribbons were actuated to produce good contrast; 3) solid supports to fix alternate ribbons; and 4) an alternative device design that produced a blazed grating by rotation of suspended surfaces (see U.S. Pat. No. 5,459,610, issued Oct. 17, 1995 to Bloom et al., entitled "Deformable Grating Apparatus for Modulating a Light Beam and Including Means for Obviating Stiction Between Grating Elements and Underlying Substrate," and U.S. Pat. No. 5,808,797, issued Sep. 15, 1998 to Bloom et al., entitled "Method and apparatus for Modulating a Light Beam." Bloom et al. also presented a method for fabricating the device; see U.S. Pat. No. 5,677,783, issued Oct. 14, 1997, entitled "Method of Making a Deformable Grating Apparatus for Modulating a Light Beam and Including Means for Obviating Stiction Between Grating Elements and Underlying Substrate".
According to the prior art, for operation of the GLV device, an attractive electrostatic force is produced by a single polarity voltage difference between the ground plane and the conducting layer atop the ribbon layer. This attractive force changes the heights of the ribbons relative to the substrate. By modulating the voltage waveform, it is possible to modulate the diffracted optical beam as needed. However, a single polarity voltage waveform can lead to device operation difficulties if charge build-up occurs on the intermediate dielectric layers between the ground plane and the conductor on the ribbons. If this dielectric charge does not dissipate quickly enough after the actuation voltage is turned off, a significant charge accumulation can occur that leads to deterioration in the performance of the device with repetitive actuation.
Charge injection and trapping into insulating dielectric films, such as silicon nitride and silicon dioxide, on semiconductors is well known to occur in various microelectronic devices. Charging effects in silicon dioxide can be minimized by proper deposition, as described, for example, in "Charge transport and transport phenomena in off-stoichiometric silicon dioxide films," Journal of Applied Physics, 54, 1983, pp. 5801-5827, by D. J. Maria et al. On the other hand, charge injection into silicon nitride can be used beneficially in non-volatile memories such as the device described by R. T. Bate; see U.S. Pat. No. 4,360,900, issued Nov. 23, 1982, entitled "Non-Volatile Semiconductor Memory Elements". Charge trapping phenomena in rolling contact micro-electromechanical actuators have also been reported by C. Cabuz et al. in "High reliability touch-mode electrostatic actuators (TMEA)," Proc. of Solid-State Sensor and Actuator Workshop, Hilton Head, S.C., Jun. 8-11, 1998, pp. 296-299.